Multi-pulse rectifier for AC drive systems having separate DC bus per output phase

ABSTRACT

An 18n-pulse rectifier for AC drive systems having a separate DC bus for each output phase is described, where n=any positive integer. The rectifier uses three separate phase rectifiers, one for each output phase of a transformer, each comprised of n six-pulse diode bridges connected in series or parallel. Each phase rectifier may be supplied with n unique sets of phase inputs from a transformer secondary winding. In some configurations, the n sets of inputs provided to each rectifier are separated by 60/n degrees of phase (when n is greater than 1), while the corresponding inputs to neighboring rectifiers are separated by 20/n degrees of phase. In a 36-pulse example, the phase offsets for the inputs provided to the rectifiers may be −25° and +5° from the transformer primary winding (for the first rectifier), −15° and +15° from the primary winding (for the second rectifier) and −5° and +25° from the primary winding (for the third rectifier). Each set of inputs may include three lines of in-phase current, and may be coupled to one of the six-pulse diode bridges.

BACKGROUND

Power plants in the U.S. produce three-phase alternating current (AC)electricity, which is then distributed, stepped down and/or rectified asneeded to produce the desired type of electricity for individualcustomers. For many applications, it is useful to rectify the ACelectricity to generate a direct current (DC) (or near-DC) output.Full-wave rectification of a three-phase input can be accomplished usinga six-pulse diode bridge for each output phase.

The six-pulse bridge, however, generates unwanted harmonic distortionthat is passed back to the power plant. Existing standards, such as IEEE519 (Institute of Electrical and Electronics Engineers), govern theamount of distortion that a given component is permitted to cause. Forexample, IEEE 519 generally limits total harmonic distortion (THD) toabout 5%, and the typical six-pulse bridge exceeds that distortionlevel.

One way of reducing this distortion is to use more than one bridge perphase. For example, some systems use 2, 3 or more bridges per phase,where using more bridges helps reduce the amount of distortion. FIG. 1shows an example of such a known configuration, in which each outputphase uses 6 six-pulse diode bridges, resulting in 36-pulses of currentper cycle. As illustrated in FIG. 1, transformer 100 includes an inputside 101A and an output side 101B. Three 36-pulse rectifiers 102A,B,Care coupled to secondary windings on the output side of the transformerto produce three output phases. As shown in FIG. 1, each 36-pulserectifier 102A is fed by six sets of secondary windings 103A. Thesecondary windings are set at ten degrees separation from one another,and are shown as being +25°, +15°, +5°, −5°, −15° and −25° offset fromthe primary winding of the input side 101A. The rectifiers in the threephases are all identical in configuration, and are each supplied withsix inputs having the same respective phase relationships (e.g.,rectifiers 102B and 102C are also fed six inputs that share the samephase relationships found in the six inputs supplied to the first phaserectifier 102A). The result is a three-phase 36-pulse rectificationconfiguration that provides a THD at around 2%, which is well below theIEEE 519 maximum.

FIG. 2 shows one of the 36-pulse rectifiers (102A,B,C) from FIG. 1 ingreater detail (as noted above, they are all identical and are providedthe same inputs). The 6 six-pulse bridges (201A-F) in each rectifier areshown as being supplied with six inputs from the transformer's secondarywindings having the phase relationship noted above (e.g., ten degreesseparation from one another). Output is available on positive terminal202A, common terminal 202B and negative terminal 202C.

The 36-pulse rectifiers used in FIGS. 1 and 2 offer significantlyreduced distortion as compared to a single six-pulse rectifier, but at asignificantly increased cost. The additional diode bridges and theirrespective inputs from the secondary windings drive up the overall costof implementing this configuration. Accordingly, there is a constantneed for improved systems that can supply comparable results at a lowercost.

SUMMARY

The following summary generally addresses many of the features describedherein, but is not intended to limit the scope of this disclosure oridentify features of greater importance to the claims herein. Althoughan improved 36-pulse system is used as an example herein, it should benoted that the techniques described are equally applicable to othermulti-pulse systems using various combinations of 6-pulse rectifiers.

The systems and features described herein relate, for example, toproviding a 36-pulse rectifier using fewer components than used intraditional 36-pulse rectifiers. The system may include a transformerhaving an input side and an output side, wherein the output sideincludes six sets of secondary windings at different phase offsets, andthree twelve-pulse rectifiers, each rectifier coupled to a unique two ofthe six sets of secondary windings. The twelve-pulse rectifiers may bemade by connecting two six-pulse diode bridges in series or parallel.

In some aspects, each rectifier is coupled to a unique two of the sixsets of secondary windings that are separated in phase by thirtydegrees. Corresponding inputs of neighboring rectifiers may be separatedin phase by ten degrees, and the six sets of secondary windings may alsobe spaced apart by ten degrees. In some aspects, a first of therectifiers is coupled to a first set of secondary windings that is 25degrees behind a primary winding of the transformer and a second set ofsecondary windings that is 5 degrees ahead of the primary winding; asecond of the rectifiers is coupled to a third set of secondary windingsthat is 15 degrees behind the primary winding and a fourth set ofsecondary windings that is 15 degrees ahead of the primary winding; anda third of the rectifiers is coupled to a fifth set of secondarywindings that is 5 degrees behind the primary winding and a sixth set ofsecondary windings that is 25 degrees ahead of the primary winding.

Additional features described herein will be addressed in greater detailbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a three-phase configuration using three 36-pulserectifiers in which each rectifier is provided with the same input(phase) as is provided to the other rectifiers.

FIG. 2 illustrates in greater detail the configuration of each 36-pulserectifier used in the FIG. 1 configuration.

FIG. 3 illustrates a three-phase configuration using three 12-pulserectifiers in which each rectifier is not provided with the same input(phase) as is provided to the other rectifiers.

FIG. 4 illustrates in greater detail the configuration of the 12-pulserectifier used in the FIG. 3 configuration.

FIG. 5 contains a table of data values showing harmonic distortions fora variety of configurations, including the one shown in FIGS. 3 and 4,as compared to requirements of IEEE 519.

DETAILED DESCRIPTION

FIG. 3 illustrates an example three-phase configuration. As shown, atransformer 300 includes an input (primary) side 301, and an output(secondary) side 302. The output side 302 may include a plurality ofsecondary windings to produce current outputs having a different leveland/or phase from that found in an input side primary winding. In FIG.3, the transformer 300 offers six sets of secondary windings 303A-F. Thesix sets are shown as being spaced apart by ten degrees, ranging frombeing 25 degrees behind the primary winding (minus 25° of set 303A) tobeing 25 degrees ahead of the primary winding (plus 25° of set 303F).

In FIG. 3, these sets are distributed, in pairs, to three separate phaserectifiers 304A-C. They are distributed such that the two sets receivedby each rectifier are spaced 30 degrees apart from one another. So, forexample, the first rectifier 304A is shown as receiving a set that is atminus 25 degrees from the primary, and a set at positive 5 degrees,totaling 30 degrees of separation. The other two rectifiers aresimilarly situated, with the second rectifier 304B receiving inputs ofminus and positive 15 degrees, and the third rectifier 304C receivinginputs of minus 5 degrees and positive 25 degrees. Aside from the 30degrees separating the two inputs for each rectifier, each rectifier'sinputs are also ten degrees apart from corresponding inputs in itsneighboring rectifier. For example, the first input to the firstrectifier, input 303A (at minus 25 degrees), is ten degrees apart fromthe first input to the second rectifier, input 303C (at minus 15degrees), which in turn is ten degrees apart from the first input to thethird rectifier (input 303E, which is at minus 5 degrees). The secondinputs to each of the rectifiers are also ten degrees apart from oneanother (e.g., plus 5 degrees, plus 15 degrees and plus 25 degrees). Aseparate DC bus is supplied to each output phase to support thisdistribution.

FIG. 4 illustrates a rectifier (304A-C) in greater detail. Asillustrated, the rectifier 304 may be a twelve-pulse rectifier comprisedof two six-pulse diode bridge rectifiers (401A-B) coupled in series.Other configurations, such as a parallel configuration, could also beused. The two inputs supplied to the rectifiers in FIG. 3 are shown asinputs 402A and 402B, and are supplied as three lines sharing the samephase. For example, the secondary winding set 303A is supplied on threelines to the input 402A of rectifier 304A, with the three linesconnected between series diodes as shown in FIG. 4. The second input303B is similarly provided on three lines to input 402B of rectifier304A. The rectifier may provide its output signals at a positive (403A),common (403B) and negative (403C) terminal. These output signals may beprovided to a DC/filtering section, or DC bus 404. Each output phase ofthis configuration may be provided with its own DC bus, as opposed tohaving a common DC bus for the three output phases.

Accordingly, the transformer 300 may offer six sets of secondarywindings set at different phase relationships, where the secondarywindings are distributed in pairs among three twelve-pulse rectifiers.By spacing the phase relationships of the secondary windings so that thesix sets are different from one another, the three twelve-pulserectifiers provide a total of 36 pulses of primary current per cycle,but have the parts count of a twelve-pulse design (using just 2six-pulse bridges per phase instead of 6).

Testing done by the inventors has shown that the design shown in FIGS. 3and 4 can offer a THD value of about 2.73%, which is comparable to(although still a bit higher than) that of the conventional 36-pulsedesign shown in FIG. 1, but which is much better than that of aconventional 12-pulse design (which supplies the same pairs of inputs toall three twelve-pulse rectifiers). The table in FIG. 5 contains dataillustrating example odd-order input current harmonics that result fromconventional 6-, 12-, 18-, 24- and 36-pulse designs (all of whichresemble FIGS. 1 and 2, but with differing numbers of six-pulse bridgesper phase—the six-pulse design uses one six-pulse bridge per phase, thetwelve-pulse design uses two six-pulse bridges per phase, etc.), ascompared to the measured harmonics generated using the FIG. 3/4 design(“MTX 36 pulse”) and the IEEE 519 limits.

The designs shown in FIGS. 3 and 4 offer improvements, but modificationsto these designs are also possible. For example, the phase offsetsbetween the secondary windings 303A-C and the primary winding 301 areshown in FIG. 3, but other offsets from the primary winding may be usedas well, so long as the internal phase relationships between rectifierinputs (e.g., thirty degrees between inputs to a single rectifier, tendegrees between corresponding inputs of neighboring rectifiers) ismaintained.

This 10-degree/30-degree relationship described above is shown for theexample in which a transformer supplies 6 sets of uniquely-phasedsecondary windings distributed across the three rectifiers in thethree-phase system. Different numbers of secondary windings can be used,and the phase relationships can be adjusted accordingly. In a systemhaving three rectifiers and 3n (with n being an integer of 1 or greater)sets of uniquely-phased secondary windings, each rectifier receives nsets of unique secondary windings. The 3n secondary windings offered bythe transformer are spaced apart at intervals of 20/n degrees. In thecase where n=2 or more, the n sets of secondary windings that aresupplied to a given rectifier are spaced apart at intervals of 60/ndegrees. So in the example illustrated in FIG. 3, the transformersupplies 6 sets of secondary windings, making n=2 for that example. Thesix sets are at 10 degree intervals (20/2), and the n sets provided toeach given rectifier are at 30 degree intervals (60/2). As anotherexample, in an 18-pulse system where the transformer provides 3 sets ofsecondary windings, the secondary windings would be at intervals of 20degrees (20/1), and each rectifier is comprised of a single six-pulsebridge supplied by a single secondary winding. As yet another example,in a 54-pulse system where the transformer provides 9 sets of secondarywindings, the secondary windings would be at intervals of 6.667 degrees(20/3), and each rectifier is provided with three (n=3) unique sets ofsecondary windings at intervals of 20 (60/3) degrees. The discussionabove uses phase intervals, and the actual choice of angles can vary aslong as the intervals are preserved. For example, two windings that areto be 10 degrees apart can be 0 and 10 degrees, 5 and 15 degrees, etc.

Further modifications and/or additions may be made as well. For example,control circuitry and smoothing filters may be added to the outputterminals (403A-C) of the rectifiers between the rectifiers and theirrespective loads.

The various features, examples and embodiments described above are notintended to limit the scope of the present application, and many of thecomponents may be divided, combined and/or subcombined with one anotheras desired. Additionally, the numeric values discussed herein (e.g.,referring to phase relationships) represent target values, andengineering tolerances will necessarily cause some implementations todeviate slightly from the exact numbers used—any such values appearingherein should be read with an understanding that such engineeringtolerance deviations may (and will) occur. Accordingly, the scope of thepresent patent should only be defined by the following claims.

1. A drive system comprising: a transformer having an input side and anoutput side, wherein said output side includes 3n (n being a positiveinteger) sets of secondary windings at different phase offsets; threex-pulse rectifiers, each of the rectifiers coupled to a different n setsof said 3n sets of secondary windings but not to the remaining 2n setsof secondary windings; and three output phase terminals, wherein saidsystem has a separate DC bus per output phase, wherein x is a multipleof six, and wherein the three rectifiers are configured to provide atotal of 3× pulses per cycle at the output phase terminals.
 2. Thesystem of claim 1, wherein said 3n sets of secondary windings are spacedat a minimum of 20/n degree intervals.
 3. The system of claim 2, whereinn>1, and each rectifier is coupled to n sets of said secondary windingsspaced apart at a minimum of 60/n degrees.
 4. The system of claim 3,wherein n=2, said system is a 36-pulse system, and said 3n sets ofsecondary windings are spaced at ten degree intervals.
 5. The system ofclaim 1, wherein n=2, and: a first of said rectifiers is coupled to afirst set of secondary windings that is 25 degrees behind a primarywinding of said transformer and a second set of secondary windings thatis 5 degrees ahead of said primary winding; a second of said rectifiersis coupled to a third set of secondary windings that is 15 degreesbehind said primary winding and a fourth set of secondary windings thatis 15 degrees ahead of said primary winding; and a third of saidrectifiers is coupled to a fifth set of secondary windings that is 5degrees behind said primary winding and a sixth set of secondarywindings that is 25 degrees ahead of said primary winding.
 6. A method,comprising: generating 3n uniquely-phased sets of voltage signals at atransformer, said 3n sets being spaced apart at an interval of 20/ndegrees, where n is a positive integer; supplying each of threerectifiers with a different n sets of said 3n sets of voltage signals,wherein each of the uniquely-phased voltage signals is supplied to onlyone of the three rectifiers, wherein each of the three rectifiers is anx-pulse rectifier, and wherein x is a multiple of six; and using saidrectifiers to provide three output phases having a total of 3× pulsesper cycle; wherein a separate DC bus exists for each of said outputphases.
 7. The method of claim 6, wherein n>1, and wherein the n sets ofvoltage signals supplied to each rectifier are spaced apart at aninterval of at least 60/n degrees.
 8. The method of claim 7, whereinsaid supplying further comprises: supplying a first one of saidrectifiers with a first one of the voltage signals that is 25 degreesbehind a transformer primary winding and a second one of the voltagesignals that is 5 degrees ahead of said primary winding; supplying asecond one of said rectifiers with a third one of the voltage signalsthat is 15 degrees behind said primary winding and a fourth one of thevoltage signals that is 15 degrees ahead of said primary winding; andsupplying a third one of said rectifiers with a fifth one of the voltagesignals that is 5 degrees behind said primary winding and a sixth one ofthe voltage signals that is 25 degrees ahead of said primary winding. 9.The method of claim 7, wherein n=2, said 3n sets of voltage signals arespaced at a minimum of 10 degree intervals, and said rectifiers are eachprovided with a unique pair of the voltage signals that are 30 degreesapart.
 10. A multi-pulse rectifier system, comprising: a transformerhaving 3n sets of secondary windings having phase offsets spaced 20/ndegrees apart, where n is an integer greater than one; a first x-pulserectifier coupled to a first group of n sets of the secondary windings,such that all of the sets of secondary windings coupled to the firstrectifier are separated by at least 60/n degrees of phase; a secondx-pulse rectifier coupled to a second group of n sets of the secondarywindings different from the first group, such that all of the sets ofsecondary windings coupled to the second rectifier are separated by atleast 60/n degrees of phase; and a third x-pulse rectifier coupled to athird group of n sets of the secondary windings different from the firstand second groups, such that all of the sets of secondary windingscoupled to the third rectifier are separated by at least 60/n degrees ofphase, wherein x is a multiple of six, and wherein the three rectifiersare configured to provide at total of 3× pulses per cycle at outputphase terminals.
 11. The system of claim 10, wherein said sets ofsecondary windings each comprise three lines of output at a commonphase.
 12. The system of claim 10, wherein said rectifiers each comprisetwo six-pulse diode bridges.
 13. The system of claim 10, wherein n=2,and corresponding inputs to neighboring ones of said rectifiers areseparated by 10 degrees of phase.
 14. The system of claim 10, wherein:sets of secondary windings in said first group are 25 degrees behind aprimary winding of said transformer and 5 degrees ahead of said primarywinding; sets of secondary windings in said second group are 15 degreesbehind a primary winding of said transformer and 15 degrees ahead ofsaid primary winding; and sets of secondary windings in said third groupare 5 degrees behind a primary winding of said transformer and 25degrees ahead of said primary winding.
 15. The system of claim 10,further comprising a separate DC bus, coupled to outputs of saidrectifiers, for each of a plurality of output phases of said system. 16.The drive system of claim 1, wherein x=12.
 17. The drive system of claim1, wherein x=one of 6, 12, or
 18. 18. The method of claim 6, whereinx=12.
 19. The method of claim 6, wherein x=one of 6, 12, or
 18. 20. Themulti-pulse rectifier system of claim 10, wherein x=12.
 21. Themulti-pulse rectifier system of claim 10, wherein x=one of 6, 12, or 18.